These days I'm dealing with Opus, and I have to say things are greatly simplified, in particular if you consider opus-tools, a set of utilities to handle opus files and traces.

One of those tools, opusrtp, can do live captures and write the interpreted payload into a .opus file.
Still, what I needed was to achieve the same result but from a pcap already existing, i.e. "offline".

So I come up with a small - quite shamlessly copy&pasted - patch to opusrtc, which is now in this fork.
Once you have a pcap with an RTP stream with opus (say in input.pcap) you can retrieve the .opus equivalent (in rtpdump.opus) with: